Low exposure concentration effects of methoprene on endocrine-regulated processes in the crustacean Daphnia magna

Methoprene is a growth-regulating insecticide that manifests its toxicity t o target organisms by acting as a juvenile hormone agonist. Methoprene simi larly may exert toxicity to crustaceans by mimicking or interfering with me thyl farnesoate, a crustacean juvenoid. We hypothesized that methoprene int erferes with endocrine-regulated processes in crustaceans by several mechan isms involving agonism or antagonism of juvenoid receptor complexes. In the present study, we evaluated this hypothesis, in part, by characterizing an d comparing the concentration-response curves for methoprene and several en dpoints related to development and reproduction of the crustacean Daphnia m agna. Our results demonstrate that methoprene has multiple mechanisms of to xicity and low-exposure concentration effects. Methoprene reduced the growt h rate of daphnids with evidence of only a single concentration-response li ne, having a threshold of 12.6 nM. Molt frequency was reduced by methoprene in a concentration-dependent manner, with a response curve corresponding t o a 2-segmented line and thresholds at 4.2 and 0.21 nM. An endpoint related to reproductive maturation, the time of first brood deposition, was also a ffected by methoprene, with a clear concentration-dependent response and a NOEC of 32 nM. Methoprene reduced fecundity according to a 2-segmented line , with thresholds of 24 and less than or equal to 0.18 nM. These results de monstrate that methoprene elicits significant toxicity to endocrine-related processes in the 5-50 nM concentration range. Furthermore, molting and rep roduction were impacted at significantly lower methoprene concentrations, w ith a distinct concentration response and a threshold of less than or equal to0.2 nM. The different concentration-dependent response from that of meth oprene could involve agonism or antagonism of various juvenoid receptor con figurations.